In order to improve the thermal efficiency of a steam turbine, it is advantageous to increase the blade length of a low-pressure final-stage moving blade. In order to increase the length of a turbine moving blade, a blade material having high specific strength is needed. However, in a steam turbine of 3600 rpm, at present, a 40-inch class is a limit for a steel-made blade, and hence a titanium alloy is used for a 45-inch class.
As a high-strength steel material for a turbine blade, Japanese Patent Provisional Publication No. 2001-98349 describes a martensitic stainless steel having a composition, in percent by weight, of 0.13 to 0.40% C, 0.5% or less Si, 1.5% or less Mn, 2 to 3.5% Ni, 8 to 13% Cr, 1.5 to 4% Mo, a total of 0.02 to 0.3% Nb and Ta, 0.05 to 0.35% V, and 0.04 to 0.15% N, the balance being Fe.
Also, as a precipitation hardened stainless steel having high strength, high toughness, and high corrosion resistance, a large number of techniques have been disclosed in the patent literature. Among these, as a stainless steel of martensite single phase, Table 1 in Japanese Patent No. 3251648 sets forth a precipitation hardened martensitic stainless steel having a composition, in percent by weight, of 0.8% or less C, 0.7 to 2.5% Si, 3.0% or less Mn, 6.0 to 7.2% Ni, 10.0 to 17.0% Cr, 0.5 to 2.0% Cu, 0.5 to 3.0% Mo, 0.15 to 0.45% Ti, 0.015% or less N, and 0.003% or less S, the balance being Fe.
A high-strength steel material for a turbine moving blade must have a high strength such that the tensile strength of a material for a blade having a 45-inch class blade length for a steam turbine of 3600 rpm is 1350 MPa or higher and the tensile strength of a material for a blade having a 50-inch class blade length is 1500 MPa or higher, a high toughness such that Charpy absorbed energy at room temperature is 20 J or higher, and a high resistance to delayed cracking (SCC). However, for a tempered martensitic stainless steel, in which the strength is controlled by quench-and-temper, as described in Japanese Patent Provisional Publication No. 2001-98349, if the tensile strength is increased to 1350 MPa or higher, a delayed crack may be generated as described later. On the other hand, for a precipitation hardened martensitic stainless steel, although high strength, high toughness, and high corrosion resistance are achieved, the precipitate of Cu, Nb or Ti alone of the background art does not provide a sufficient strength as compared with a required value of 1500 MPa or higher. For the technique described in Japanese Patent No. 3251648, although high strength and high toughness are achieved by decreasing the crystal grain size, it is difficult to obtain a fine grain structure in a thick portion such as a turbine blade root portion, which presents a problem of insufficient strength and toughness.